There have conventionally been used means for modifying the properties of a surface of a base material to impart water repellency, an anti-contamination property, and the function of aligning a liquid crystal thereto. As an example of such means, there is a method of coating the base material with a solution containing a trichlorosilane-based compound, a silane coupling agent, or the like to cause chemical adsorption of a coating component to the surface of the base material. In accordance with the method, solute molecules as the coating component are strongly bonded to the base material by chemical adsorption, so that a high-resistance coating is formed. If the surface of the base material has a reduced number of active hydrogens such as hydroxyl groups, however, a coarse coating having a low density of adsorbed molecules is formed. With such a coating, the object of modifying the surface properties cannot be attained satisfactorily.
With respect to a base material having a low density of active hydrogens, therefore, there has been adopted a method of bonding and fixing a coating film to the base material via an underlying layer by using the technology of preliminarily coating a sol-gel solution containing SiO2 or the like as a main component, sintering the sol-gel solution to form the underlying layer having a high density of active hydrogens on the surface, and applying a coating solution containing a silane coupling or the like. The method increases the resistance of the coating compared with a method of directly applying the base material with the coating solution. However, a functional film formed by using the prior art technology is not satisfactory in terms of adhesion to the base material, uniformity of the coating, resistance, and the like so that a further improvement is expected.
There can also be considered a method of preliminarily mixing SiO2 in a solution containing a silane coupling agent, a trichlorosilane-based compound, or the like and coating the mixture directly onto the base material to simplify the operation of forming the underlying layer and increase the density of active hydrogens. In accordance with the method, however, the silane coupling agent, the trichlorosilane-based compound, or the like is segregated at the interface between the substrate and the functional film and therefore the adhesion between the substrate and the functional film cannot be increased.
Since the resistance of a functional film is also dependent on the resistance of the underlying layer, an underlying layer with resistance higher than that of the SiO2 film is desired for a functional film with higher resistance.